Running Blender 2.8 and 2.7 on same PC
If you want multiple blender versions (I have three) to play nicely together you have to download them as zips and then extract them instead of just running the installer. 

Alternately you can put your older blender installations into zip files, run the installer, and then unzip the old versions again back to where they were. I have used this method myself in the past but it's so much easier to just get the zip for the latest version to start with.

I got this answer in part from here.